Injection molding of conventional rubber is carried out by blending additives with the rubber, kneading the blend and curing it after injection into a mold. Such a process entails such disadvantages as the necessity of using a special molding machine, long cycle times and the need to carry out a number of complicated processing steps. Similar disadvantages occur in extrusion molding, and these disadvantages have made it impossible to carry out smooth production of rubber products. Thus, it has been suggested that rubber be replaced with materials which can be processed without curing, but that have properties similar to those of rubber.
This suggestion has been put to practice with materials having rubber-like properties. Among the materials which have been used are soft plastics such as soft vinyl chloride resins, ethylene-vinyl acetate copolymers, and low density polyethylenes. Although these materials have good processability and high flexibility, they suffer such drawbacks as low heat-resistance and low rebound elasticity which severely limits their use.
In order to improve the heat-resistance and mechanical strength of soft plastics, soft plastics have been blended with a plastic of a high melting point polymer such as high density polyethylene or polypropylene. This blending, however, does not result in a good product because it causes a loss of flexibility, and further, when a thick product is molded from the blended material sinkmarks are apt to occur. Moreover, the prior art blends exhibit stress-whitening when subjected to an impact. Recently, attention has being given to "thermoplastic elastomers", a group of materials that have properties which fall between those of cured rubbers and soft plastics.
Olefinic thermoplastic elastomers are already known. For example, U.S. Pat. No. 4,087,486 to Fielding et al. provides a thermoplastic composition that comprises from 5 to 30 parts by weight of a saturated ethylene-propylene rubber, 95 to 70 parts by weight of a crystalline propylene homopolymer, and from 0.01 to 0.2 parts by weight of an organic peroxide per each 100 parts by weight of the total concentration of saturated ethylene-propylene rubber and polypropylene. These compositions, which are said to have improved knit-line properties, are prepared by first blending the aforementioned components and then adding the blend to an extruder.
U.S. Pat. No. 4,140,732 to Schnetger et al. provides another thermoplastic rubber composition which purportedly exhibits improved tensile strength, elongation and tear propagation resistance values. The thermoplastic compositions disclosed in Schnetger et al. comprise mixtures of either partially crosslinked ethylene-propylene or ethylene-propylene-diene sequential polymers and a polyolefin resin containing olefinic monomers such as ethylene, propylene, 1-butene and the likes thereof. The partial crosslinking occurs either during or after the mixing of the sequential polymers and the polyolefin resins.
Another thermoplastic composition that can be used for injection molding applications is disclosed in U.S. Pat. No. 4,829,125 to Yeo et al. The thermoplastic compositions disclosed in Yeo et al. are prepared by first blending an olefin copolymer rubber and a crystalline polypropylene polymer in a ratio of 1:1.01-0.5 to prepare a preblend and then melt blending 10-75 parts by weight of the preblend and 90-25 parts by weight of the crystalline polypropylene polymer in the presence of an organic peroxide in an extruder.
U.S. Pat. Nos. 3,758,643 (now reissued as U.S. Pat. No. Re 30,405) and 3,862,106 to Fisher et al. also describe thermoplastic blends that can be used as molded or extruded articles. Specifically, the thermoplastic blends disclosed in these Fisher et al. patents comprise a partially crosslinked rubbery copolymer of ethylene and at least one other copolymerizable monoolefin of the formula CH.sub.2 .dbd.CHR where R is an alkyl radical having from 1 to 12 carbon atoms and an non-crosslinked resinous polyolefin such as polypropylene wherein the weight ratio of the rubbery copolymer to uncrosslinked resinous polyolefin is from 10:90 to 90:10. The partial crosslinking of the rubbery copolymer occurs prior to blending with the non-crosslinked resinous component by the action of a curative to obtain a rubbery copolymer having a gel content of at least 30 percent but less than 90 percent by weight.
Despite the current state of the art, none of the references noted hereinabove disclose a process for preparing high flow rate thermoplastic compositions, which exhibit reduced stress-whitening and improved impact strength, by melt mixing above the decomposition temperature of a peroxide, a mixture comprising a high propylene content polymeric component and a non-crosslinked elastomeric component which has been intimately precontacted with a peroxide at a temperature below the melting point of the polymer.
In contrast to the method disclosed in the present invention, all of the above-identified references require that the polymers or the polymer blends be partially or fully visbroken. The term "visbroken" as used in the prior art means heating a polymer or mixture of polymers to effect degradation via thermal cracking to produce a product having segments of lower molecular weight, evidenced by a greater flow rate. Consequently, the thermoplastic compositions described in the prior art do not exhibit the unexpected viscosity upturn in their dynamic melt rheological data at frequencies of less than about 1.0 rad/sec which is exhibited by the compositions of the instant invention.